1. Field of the Invention
The present invention relates to an interference device and an interference position measuring device for detecting a position fluctuation of an object in a non-contact manner. Particularly, the invention is suitable for a micro interference displacement meter which achieves resolution and accuracy of submicron order by applying a light interfering phenomenon, a machine tool, an assembly adjusting device or the like utilizing the displacement meter.
2. Related Background Art
An interference device which applies a laser is widely utilized as a length measuring device with high accuracy. Generally, such a device requires absolute accuracy and uses a gas laser with a stable wavelength. Further, recently, a device using a semiconductor laser which has been characterized in compactness and simplicity as a simple interference device.
FIG. 1 is a schematic diagram of a conventional interference device using a semiconductor laser as a light source. In FIG. 1, a laser light flux 20 emitted from a semiconductor laser 1a is converted into a parallel light by a collimating lens 2a so as to enter a polarized beam splitter 4 and is divided into a measurement light 20a and a reference light 20b. The measurement light 20a transmits through a ¼λ plate 5b and is converted into a condensed light flux by a condenser lens 6 so as to be condensed on a reflecting surface 7 of an object to be measured 7a. Meanwhile, the light flux reflected by the polarized beam splitter 4 transmits as the reference light 20b through a ¼λ plate 5a and is reflected by a reference mirror (reference surface) 8. The light fluxes which are reflected by the object to be measured 7a and the reference mirror 8, respectively, transmit again through the ¼λ plates 5a and 5b, and the reference light 20b transmits through the polarized beam splitter 4, and the measurement light 20a is reflected to be a multiplexed light flux 21 and enter a ¼λ plate 5c. Since only polarization information of a return light of the measurement light 20a in the multiplexed light flux 21 is modulated, the light flux which has transmitted through the ¼λ plate 5c becomes a linear polarized light which rotates. After that, the light flux enters a nonpolarized light beam splitter 10 so a to be divided into two light fluxes 24a and 24b. Thereafter, the light fluxes transmit through polarizing plates 11a and 11b, optical axes of which are tilted 45° from each other, respectively, so that sine wave signals, phases of which are different by 90° from each other (hereinafter, called the A phase signal and B phase signal) as shown in FIG. 2, are generated on sensors 12a and 12b. Since a polarizing direction of the light flux 20a rotates due to displacement of the object to be measured 7a in a direction of optical axis La, one cycle sine signals can be obtained at sensors 12a and 12b in λ/2 according to the displacement of the object to be measured 7a. 
The interference device shown in FIG. 1 is suitable for measuring a rotating axis or the like which is manufactured by mechanical working, for example. When a laser beam is emitted to a scattered portion of the reflecting surface 7 on the object to be measured 7a, a speckle pattern, which is caused by coexisting of a light flux with various phases, is generated in a reflected light. The speckle pattern is a granular pattern of a light, but in the case where the speckle pattern is interfered with a reference light, since interference signals of the speckles have random phases, interference light signals on a sensor are averaged so as to be in a so-called dropout state. The A phase and B phase sine wave signals from the sensors 12a and 12b are calculated into a displacement of the object to be measured 7a according to counted values obtained by counting the A phase signal and the B phase signal by means of a counter and phase information of the A and B phases. Namely, at the time of dropout, since a counter value which is necessary for calculation of displacement information is not updated, when the object to be measured 7a displaces not less than a counter operating distance at the time the signal is recovered, that displacement becomes an error. In the case where the object to be measured is a rotary object, dropout occurs on one portion every time, and since a motion of the axis is mainly axes deviation which synchronizes with the rotation, the axis shows approximately similar action at the time of dropout. Therefore, an error at the time of the dropout is accumulated in every cycle and displacement occurs infinitely. As mentioned above, the measurement of a rotary object has a problem that an error is accumulated infinitely.